JP2804361B2 - Semi-solid metal production method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a solid in which non-dendritic primary crystals are dispersed in a molten metal.
The present invention relates to a method for stably producing a liquid metal mixture (simply called semi-solid metal for simplicity).

(Prior Art) Methods for producing semi-solid metal include, for example, JP-B-56-20
As disclosed in Japanese Unexamined Patent Publication No. 944, a molten metal (generally, an alloy) is vigorously stirred while being cooled by a high-speed rotation of a stirrer in a cylindrical cooling and stirring tank, and the dendrites being formed in the molten metal. The crystal form is changed to a rounded form in which the branches disappear or shrink, and this is dispersed to form a slurry-like semi-solid metal which is mixed in the metal melt,
Continuously discharge from the bottom nozzle of the cooling and stirring tank, or repeatedly discharge and re-inject each time the cooling and stirring process of the slurry-like semi-solid metal is completed without continuous discharging. Is also known.

As a stirring method during the cooling, in addition to the mechanical stirring described above using a stirrer, an electromagnetic stirring method for electromagnetically stirring a molten metal in a cooling stirring tank is also known.

Although the production of semi-solid metal by these methods are possible, any of the solid phase rate to a target In the method (f _s) and the solid-liquid rate of increase of the solid phase rate per unit time in the coexistence (Easy Flow of the semi-solid metal formed by the average value of the velocity change per unit distance of the fluid (referred to simply as the shear strain rate for simplicity), which depends on the stirring speed of the fluid. Difficult to produce a stable semi-solid metal, such as inability to discharge or solidification clogging due to flow stoppage of the semi-solid metal in the tank even at the same solid fraction (f _s ) It became clear that.

(Problems to be Solved by the Invention) The fluidity of a semi-solid metal generally becomes worse when the solid phase ratio, which is expressed as a ratio of the volume of the solid phase metal to the total volume of the slurry semi-solid metal, increases, Above a certain solid phase ratio, usually above 0.65, it is impossible to discharge or transfer from the semi-solid metal manufacturing device to the next step multi-stage semi-solid metal manufacturing device or casting device, or to the holding device, or the processing device, Discharge is impossible or a problem occurs due to stoppage of flow or blockage solidification in the semi-solid metal production apparatus.

In addition, when producing semi-solid metal with continuous cooling, the fluidity increases as the solidification rate during solidification increases and the shear strain rate decreases, even when the solid fraction (f _s ) is 0.65 or less. Turned out to be worse. That is, in order to perform stable production of semi-solid metal or stable discharge and transfer to a multi-stage semi-solid metal production device or casting device, holding device and processing device in the next process, the solid phase ratio of semi-solid metal,
Flow rate (viscosity) as well as solidification rate during solidification
Agitation of the shear strain rate corresponding to the solid fraction and the cooling rate of the semi-solid metal under cooling,
Alternatively, it is necessary to perform a cooling rate commensurate with the shear strain rate and to appropriately manage fluidity.

(Means for Solving the Problems) Production experiments of semi-solid metal in a slurry state were performed at various solidification speeds and stirring conditions, and the solid phase ratio (f _s ) and the solidification rate that could ensure the fluidity of the semi-solid metal were confirmed. The relationship between the speed and the shear strain rate is elucidated, and the required shear strain rate is secured by selecting the stirring rate depending on the solidification rate of the semi-solidified metal, and the solid phase ratio ( f _s) by changing the, or settings and the solid fraction of the solidification rate depending upon which the shear strain rate (f _s) above problems by changing the are those that can be advantageously solved.

That is, the present invention injects the molten metal into the cooling and stirring layer,
Making the slurry semi-solid metal of the solid-liquid coexisting state giving stirred at that cooling solidification process, the solid phase ratio as operating conditions (f _s) and the average solidification rate R and the values of fluidity index according shear strain rate η Performing a stirring and cooling operation within a range satisfying the following expression (1), and discharging the slurry-like semi-solid metal after the operation from a cooling and stirring tank.

(SL) η = a / 2 (1 / f s -1 / f scr) ≦ 10 ... (1) a = 35000 · R 0.5 · -1.7 [-] ^{_{f scr = 0.65-1.4 · R 1/3 ·}} - ^1/3 [-] f _scr > f _s η; fluidity index [-] f _s ; solid fraction of semi-solid metal in slurry (target solid fraction) [-] R; solidification of raw molten metal Average solidification rate [% · s ⁻¹ ]; shear strain rate [s ⁻¹ ] determined by stirring speed.

Here, as described above, the average solidification rate R [% · s ⁻¹ ] is defined as the rate of increase in the solid fraction per unit time in the solid-liquid coexistence state. For example, when the molten metal is cooled from the liquidus temperature to the solidus temperature to produce a semi-solidified metal having a solidus fraction (f _s ), The solidification rate up to the temperature is not always the same (the solidification rate is larger at the start of cooling and becomes smaller as the temperature approaches the solidus temperature). In the cooling during this for solidification rate R from the liquidus temperature up to the solidus temperature, the solid phase ratio to target solid fraction increase in until the (f _s) f _s (here The indicated f _s is actually the same as the target solid fraction (f _s ))
During this period of time determined by R = f _s / Δt as Delta] t [% · s ^-1].

Further, the present invention is based on the fact that the stirring and cooling operation of the slurry-like semi-solid metal is performed by successive repetition in the cooling and stirring tanks provided in multiple stages, and here, the first stage of the cooling and stirring tank is operated at a relatively high solidification rate. It is preferable that the operation is sequentially performed at a lower solidification rate in the subsequent cooling and stirring tank, and that the molten metal is an aluminum alloy.

(Operation) The inventors conducted experiments on the production of slurry-like semi-solidified metals using molten metals of alloys having various compositions at various solidification rates and stirring conditions, and obtained an index of the fluidity (viscosity) of the semi-solidified metal. The relationship between the value η, the liquid limit solid fraction fscr indicating the limit of fluidity, the solidification rate R [% · s ⁻¹ ], and the shear strain rate [s ⁻¹ ] was investigated, and the relation shown in the equation (1) was obtained. I got FIG. 1 shows an example of the result. In other words, the fluidity index value η is the solid phase fraction (f _s ) and the liquidity critical solid phase fraction (for simplicity, simply referred to as the critical solid fraction f _scr ) indicating the fluidity limit of the slurry-like semi-solidified metal. F _scr and a are functions of the average solidification rate R [% · s ⁻¹ ] and the rate of shear strain during solidification of the molten metal. There, η = a / 2 (1 / f s -1 / f scr) a = 35000 · R 0.5 · -0.7 [-] ^{_{f scr = 0.65-1.4 · R 1/3 ·}} 1/3 [-] relationship And found that the fluidity can be stably secured by satisfying the relationship of η ≦ 10.

Here, (f _s ) is the solid fraction obtained from the equilibrium diagram based on the temperature measurement value at the outlet side of the cooling and stirring tank, and f _scr > f _s .

According to this result, it is preferable that the fluidity index value η of the semi-solid metal to be discharged to the next step after finishing the cooling and stirring in the production of the slurry-like semi-solid metal is 10 or less, preferably 5 or less.

That is, the solid fraction in order to ensure the fluidity of the semi-solid metal (f _s) and the minimum shear strain rate depending upon which the solidification rate is determined to be discharged.

However, it is necessary to increase the solidification rate in order to reduce the crystal grain size of the slurry-like semi-solid metal, but if the solidification rate is increased, the fluidity is reduced as described above, so the shear strain rate is inevitably increased. solid fraction of slurry semi-solid metal discharged from the outlet side of the or increasing cooling agitation tank (f _s)
Need to be lowered.

Therefore, when producing a semi-solid metal with a high solidification rate in which the solidification rate is increased and the crystal grain size is reduced, a high shear strain rate can be obtained by a device or a multi-stage device, and the former device has a high solidification rate. Produces a semi-solid metal with a low solid phase rate at the solidification rate, and transfers it to the semi-solid metal production equipment with a low solidification rate at the later stage of the next process, and raises the solid phase rate to increase the solid phase rate of the fine crystals. Can be produced.

Thus, the above problems are solved, and a semi-solid metal having a target solid fraction (f _s ) from a low solid fraction to a high solid fraction is discontinuously or continuously stably produced. Is now possible.

(Example) Example 1 A molten metal of an Al-4.5% Cu alloy was poured into a semi-solid metal manufacturing apparatus shown in FIG. 2, and a stirrer was set at 600 rpm (shear strain rate = 300 /
The average solidification rate during solidification in the cooling bath while stirring in s)
After cooling at 3.0% · s ^-1 , the temperature of the semi-solid metal discharged at the outlet of the bottom nozzle of the device was continuously measured, and the solid phase fraction (f _s ) converted from the temperature based on the equilibrium diagram was 0.25. As a result, the semi-solid metal was continuously and stably produced, and was discharged to the processing device in the next step without causing a stagnation of the flow.

Example 2 A molten metal of an Al-10% Cu alloy was poured into the semi-solid metal producing apparatus shown in FIG. 3, and the stirring bar was rotated at 600 rpm (shear strain rate = 280 /
The average solidification rate during solidification in the cooling bath while stirring in s)
Cooled in 0.45% · s ^-1, results fraction solid of temperature conversion of semi-solid metal of the internal stirring tank (f _s) is to produce a semi-solid metal of 0.35, it can be production of semi-solidified metal a flowable Was.

Example 3 In the first stage of the semi-solid metal production apparatus shown in FIG.
Inject the molten metal of Al-4.5% Cu alloy and cool the average solidification rate during solidification in the cooling tank at 23.0% · s ^-1 while stirring the stirrer at 900 rpm (shear strain rate = 450 / s) and, bottom solid phase ratio of the nozzle exit temperature conversion apparatus (f _s) is discharged transferring the semi-solid metal of 0.11 downstream of the device, the average solidification rate during solidification in the subsequent cooling bath 0.20% · s ^{- After} cooling at ¹ and discharging the semi-solid metal having a solid phase ratio (f _s ) of 0.47 in terms of the bottom nozzle outlet temperature, the production and discharge of the semi-solid metal could be continuously and stably performed.

In FIGS. 2 to 4, 1 is a heat retention tank, 2 is a cooling and stirring tank, 3 is a stirrer, 4 is a drive shaft, 5 is a ladle, 6 is molten metal supplied, 7 is cooling water, 8 is a water cooling jacket, 9 Is a semi-solid metal in a slurry state, 10 is a thermocouple for temperature measurement, 11 is a discharge nozzle,
12 is a slide gate, 13 is an induction heater or 18 is a tundish, 19 is a heater coil, and particularly in FIG. 4, 14 is a continuous semi-solid metal production apparatus in the former stage, 15 is a transfer pipe, and 16 is a continuous semi-solid metal continuous. The production equipment, 17 is a twin-roll caster, and 20 is a ceramic coating.

The solidification rate in each of the above embodiments was controlled by changing the material of the cooling tank inner wall, the amount of cooling water, and the gap between the cooling tank inner wall and the stirrer.

Table 1 summarizes the results of the other examples in addition to the above examples.

FIG. 5 shows a change in the discharge rate over time during the production of the semi-solid metal of Example 1 according to the present invention, together with a comparative example. In the example of the present invention, the discharging speed is stable, but in the comparative example, the discharging is stopped due to the fluctuation of the discharging speed and the clogging in the tank.

(Effect of the Invention) The method for producing a semi-solid metal according to the present invention exhibits the following effects.

(1) Even a semi-solid metal continuous production apparatus with a high solidification rate at which the semi-solid metal has a poor fluidity and is easily clogged in the apparatus can be stably and continuously produced and discharged.

(2) the solid phase rate (f _s) it is possible to manufacture stably and continuously the high fraction solid semi-solid metal, such as 0.6.
(3) It is possible to stably produce a semi-solid metal having good fluidity even with a discontinuous semi-solid metal production apparatus.

(4) Therefore, the semi-solid metal is discharged from the semi-solid metal manufacturing apparatus, and an accident such as blockage in the apparatus due to transfer to a subsequent apparatus or discharge and transfer to a holding apparatus, a casting machine, and a processing apparatus in the next process. And stable operation is possible.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a solidification rate, a shear strain rate and a solid phase rate at which the fluidity of a slurry-like semi-solid metal becomes constant. FIG. 2 shows a semi-solid metal continuous production apparatus used in an embodiment of the present invention. FIG. 3 is an explanatory view showing a discontinuous production apparatus of semi-solid metal used in the same embodiment, and FIG. 4 is an explanatory view of a multi-stage semi-solid metal continuous production apparatus for high solid phase ratio. FIG. 5 is a comparison graph of the discharge speed and the discharge solid phase ratio with respect to the discharge elapsed time in Example 1. DESCRIPTION OF SYMBOLS 1 ... Insulated tank, 2 ... Cooling and stirring tank 3 ... Stirrer 4 ... Drive shaft 5 ... Ladle, 6 ... Molten metal supply 7 ... Cooling water, 8 ... Water cooling jacket 9 ... Half Solidified metal, 10 ... Thermocouple for temperature measurement 11 ... Discharge nozzle, 12 ... Slide gate 13 ... Induction heater, 14 ... Front-end semi-solid metal continuous production equipment 15 ... Transfer pipe, 16 ... Rear half Solidification metal continuous production equipment 17 Twin roll casting machine 18 Tundish 19 Heater coil 20 Ceramic coating

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Ryuji Yamaguchi 1 Kawasaki-cho, Chiba-shi, Chiba Pref. JP, A)

Claims (4)

(57) [Claims] When a molten metal is poured into a cooling and stirring tank, and stirring is performed during the cooling and solidification process to produce a slurry-like semi-solid metal in a solid-liquid coexistence state, the solid phase ratio (f _s ) is determined as an operating condition. The stirring and cooling operation is performed in a range where the value of the fluidity index (η) according to the average solidification rate (R) and shear strain rate () satisfies the following formula (1), and the slurry-like semi-solidified metal that has passed through the operation is obtained. Is discharged from a cooling and stirring tank. (SL) η = a / 2 (1 / f s -1 / f scr) ≦ 10 ... (1) a = 35000 · R 0.5 · -1.7 [-] ^{_{f scr = 0.65-1.4 · R 1/3 ·}} - ^1/3 [-] f _scr <f _s η; fluidity index [-] f _s ; solid fraction of semi-solid metal in slurry (target solid fraction) [-] R; solidification of raw molten metal Average solidification rate [%
s ^-1 ]; shear strain rate [s ^-1 ] determined by stirring speed 2. The method for producing a semi-solid metal according to claim 1, wherein the stirring and cooling operation of the slurry-like semi-solid metal is sequentially repeated in a cooling and stirring tank installed in multiple stages. 3. The high-solids fraction having a high solid phase ratio according to claim 2, wherein the first cooling-stirring tank operates at a relatively high solidification rate, and the second cooling-stirring tank operates at a successively lower solidification rate. Method for producing solidified metal. 4. The method for producing a semi-solid metal according to claim 1, wherein the molten metal is an aluminum alloy.